Process of making a plate oxide rectifier



United States Patent PROCESS OF MAKING A PLATE OXIDE RECTIFIER Ronald B. Howes and Robert F. Gill, Jr., Cincinnati, Ohio, assignors to Jack F. Koons, Jr., Cincinnati, Ohio Applicafion March 20, 1956, Serial No. 572,654

17 Claims. (Cl. 1486.3)

This invention relates to dry electrical rectifiers of the plate type having an oxide layer which serves to pro- -making such improved rectifier units which method is conducive to reliable production under practicable tolerance requirements with minimum investment for production equipment and from materials which are in adequate supply. Further features of the improved rectifier reside in reduced capacitance effect, reduction in volume and weight of the units, low noise level, and enhanced resistance to peak surges and the ability to self-remedy minor damage to the oxide film. Additional features and advantages of the invention will become apparent as the description of the improved rectifier proceeds.

The method of making the rectifier units, their struc ture, and the performance characteristics resulting from the several process steps herein described are illustrated and demonstrated by reference to the drawings, in which:

Fig. l is a process chart indicating the several principal steps of the method of making a rectifier in accordance with the invention, certain steps and sequences being optional, as is more fully set forth below, and

Figs. 2 and 3 are graphical representations of performance characteristics of rectifier elements more fully described hereinafter.

Briefly, the invention contemplates a rectifier element comprising a plate of commercially pure titanium having a film of the semiconductive oxide of the metal upon a surface thereof, this film being formed in situ by exposure of the titanium surface to the action of steam under closely controlled conditions after which the primary oxide film so formed may be supplemented and perfected by chemical and/or heat treatment to enhance the electrical and mechanical characteristics of the recti fier element.

To facilitate ready and complete understanding of the invention, it is here pointed out that it comprises two aspects, one dealing with improvements in the formation of the primary titanium oxide film to form a rectifier plate without further processing of the film so formed, the other dealing with the treatment of the primary film prior to the application of the counter-electrode structure in the production of a rectifier plate having improved mechanical and electrical performance characteristics. Since the steaming process by which the primary film is formed is employed in all forms of the rectifier contemplated by this invention, it will first be described in detail, after which the supplementary treatment processes will be described.

' Commercial grade, as distinguished from chemically pure, titanium metal is employed. It appears that the 2,825,668 Patented Mar. 4, 1958 presence of impurities in limited amount tends to catalyze formation of the desired oxide film. The following, given by way of example, is the chemical analysis of a commercial grade of titanium which is suitable for the purposes of the invention:

The titanium content of the metal should preferably lie between 97.0 and 99.5 percent of the weight of the material, although commercial titanium in which the titanium content lies between 95.0 and 99.6 percent gives satisfactory results. Sheet metal 0.022 inch thick is suitable and other thicknesses may be used in accordance with mechanical requirements and cost considerations.

The sheet titanium is cut into suitable shapes and sizes with appropriate mounting holes after which a surface is prepared for treatment by thoroughly cleaning the same, desirably including a moderate nitric-hydrofluoric acid etch followed by thorough rinsing.

Following such preliminary preparation of the titanium plate by procedures which are generally known, the first essential step in the preparation of titanium oxide rectia flats in accordance with the invention is the formation of a semiconductive film of titanium oxide upon a surface of the plate. Development of this film, referred to herein as the primary oxide film in view of the subsequent deposition of additional semiconductive oxides in one aspect of the process of the invention, is accomplished by exposure of the titanium surface to pure (i. e., substantially free from other gases) superheated steam under carefully controlled conditions of time, temperature and velocity of movement of the steam over the surface. The water vapor appears to act, in the presence of the impurities contained in the commercial grade of titanium used, as a combination oxidant-reductant to develop an oxide film having an oxygen content which renders it peculiarly effective as an 'n-type semiconductor for the formation of the rectifying barrier. If the steaming process is conducted under very carefully controlled conditions within critical ranges of time, temperature and velocity of flow, it is possible to prepare a rectifier which is suitable for many purposes Without the necessity of subsequent treatment of the primary oxide film other than the application of a suitable counter-electrode. Rectifier plates having steam-developed primary films produced outside of the critical control ranges, and especially without remedial post-steaming treatment of the oxide film, exhibit mechanical and performance defects which render them unsatisfactory for most rectifying purposes. For example, steam oxidation temperatures below about 1400 F. have been found to produce films which are so lacking in uniformity that they are not satisfactory for use as area rectifiers although local spots may be capable of efiecting a relatively high degree of rectification. Steam oxidation at temperatures above about 1550 F. result in increasingly brittle films which are susceptible to failure under ordinary conditions of handling and use of rectifier-s.

It will be understood from the foregoing that while it is possible to produce satisfactory rectifiers by the steam development of an oxide film upon the titanium plate without post-steaming treatment or addition to the semiconductive oxide film, substantial improvements are efiect-- ed by such subsequent treatments or additions. Even primary oxide films of inferior quality, due to departures from the critical conditions prescribed for the steaming process, can be converted into satisfactory rectifying elements by treatment of the primary film in accordance with the procedures herein described prior to application of the counter-electrode.

Steam oxidation of the prepared surface of the titanium plate is preferably carried out in a furnace having a long tube in which the plates to be treated can be arranged and through which the steam may flow under carefully controlled conditions of temperature and velocity. Means must be provided for preheating the metal plates to avoid the condensation of moisture upon their surfaces and for the gradual cooling of the finished plates to avoid strain or damage to the oxide film. For highest quality rectifier films, the temperature of the plates during the steaming process should be maintained in the range between 1485 F. and 1550" F. The permissible range may be considered to be from about 1400 F. to about 1550 F. The rate of flow of the steam over the plate surfaces must be controlled within the range of about 65 feet per minute to about 250 feet per minute. The time required for the exposure of the titanium surfaces to the steam for proper development of the oxide film is from one-half to four hours. Only sulficient superatmospheric pressure is maintained to make sure that the steam is not contaminated by air or other gases.

As a specific example of a controlled steam oxidation procedure productive of oxide coated titanium plates suitable for rectifier use without further treatment of the oxide film, the following specification is prescribed: Properly cleaned and prepared titanium plates 1% x 1%" are placed on a suitable support in the stainless steel furnace tube five inches in diameter and forty-eight inches long-in which the temperature is 1500 F. Steam is supplied to one end of the tube at the rate of 0.2 pound per minute, resulting in a velocity of about 120 feet per minute through the length of the furnace tube. Temperature and steam flow are maintained at the established levels for a period of 120 minutes, after which the supply of steam is discontinued and the plates are permitted to cool gradually. The plates so treated may then be provided with a suitable counter-electrode upon the oxide film, or they may be subjected to further heat or supplemental oxidation treatment prior to the application of the counter-electrode.

As the process chart of Fig. 1 shows, the steam-developed primary oxide film may be subjected to a poststeaming heat treatment or to a supplemental oxide deposition, or to both of these procedures in either sequence, prior to application of the counter-electrode and finishing of the rectifier assemblies.

The principal purpose and effect of the post-steaming heat treatment is to improve the electrical and thermal stability of the finished rectifier. As the process chart indicates, such heat treatment is not indispensible, and may be omitted, but it is desirable. In general, the permissible operating voltage of the rectifier is increased by the post-steaming heat treatment, the forward resistance is somewhat diminished, and power loss and operating temperatures are diminished due to reduction of capacitance of the rectifier in the electrical circuit. Since the post-steaming procedures improve the rectification performance of the oxide film, somewhat greater latitude may be permitted in steaming conditions when such further treatment is to be employed. For example, an adequate temperature range for the formation of the primary oxide film is from about 1200 F. to 1600 F.

The post-steaming heat treatment consists of heating the rectifier plates in a suitable inert atmosphere (e. g. carbon dioxide or argon) which does not effect chemical change in the primary oxide film to a temperature between about 300 'F. and 600 F. for a period of several hours; say, from two to eight hours. For example, the

plates may be maintained at a temperature of about 340 F. for a period of four hours.

Following this heat treatment, the counter-electrode may be applied and the rectifier otherwise finished, or the primary oxide film may be subjected to further chemical treatment for the addition of a supplemental semiconductive oxide deposit. A solution of a suitable salt is used for this treatment. Any salt which is subject to thermal decomposition to an oxide which is an intrinsic semicenductor may be used. For example, the conveniently soluble nitrates, sulfates, halides, hydroxides and acetates of nickel, cobalt, copper, iron, barium, cerium, uranium, or indium have been found to be suitable. The process may be carried out either by quenching the heated plates in a suitable solution of one ,or more of these salts with a suitable wetting agent, as by dropping the plates from the post-steaming heat treatment above described into the solution, and thereafter heating the plates to drive off moisture and complete the oxide deposition, or the supplemental oxide deposition may be formed by wetting the oxide film as by immersion of the plates in the solution at, say, room temperature, and thereafter heating the plates to a temperature sufliciently high to not only evaporate the water from the surface of the film but to decompose the salt to the oxide. In either case, the finished product is a dry surface of titanium and supplemental oxides from which any loose oxide particles may be removed by light brushing after which the counterelectrode may be applied to this composite film surface.

It has been found that nickel nitrate is most convenient to use for the supplemental oxide deposition and rectifier plates treated with this salt have excellent performance characteristics. Solution concentrations are not critical, but a range of from A molar to one molar has been found to be most satisfactory, a molar solution being preferred, especially when the quench technique is employed.

The overall resistance of the finished rectifier is increased by the addition of the supplemental oxide dep0sition, while other important qualities are greatly improved. The greater uniformity of the composite oxide film results in a substantial increase in the current carrying capabilities of the rectifier plates. High inverse peak voltages obtainable with such rectifiers demonstrate a closer approach to a complete rectifying barrier.

If the supplemental oxide deposition has been effected directly upon the primary oxide film, without interposition of a post-steaming heat treatment, the composite oxide unit may be subjected to heat treatment as above described to further improve the mechanical and per formance characteristics, after which the counter-electrode is applied to the film and the rectifier assembly otherwise completed.

The process by which the counter-electrode is applied to the oxide film forms no part of this invention. Any suitable technique may be followed in the application of a coating of silver in intimate contact with the oxide. For example, the counter-electrode may be deposited by well known vacuum-vapor techniques and a suitable gross counter-electrode of silver, copper, bismuth, or nickel may be applied for greater strength, if desired. The finished rectifier elements may be encapsulized or may be assembled in open construction, as desired.

Rectifiers constructed in accordance with the invention exhibit very substantial advantages over known types of rectifiers. Some of the principal advantages may be enumerated as follows:

1) The improved rectifiers may be employed for the rectification of currents of considerably higher frequencies. They can handle frequencies as much as 500 percent higher than the maxima for selenium cells.

(2) The improved units operate satisfactorily over a wider range of ambient temperatures. They have been used at temperatures ranging from F. to 425 F.

(3) Limited damage to the rectifying layer due to instantaneous surges or impulses will usually heal itself.

(4) The rectifiers have a high current carrying capacity-as much as 12 amperes per square inch of rectifying surface.

(5) The units enjoy longer useful service life and greater flexibility of application.

(6) They have a high level of operating efiiciency.

(7) The rectifier elements exhibit desirable high inverse voltage characteristics.

(8) The forward resistance is lower, resulting in lower temperature operation and the possibility of using a larger number of plates to achieve the desired direct current output voltage.

(9) The equipment cost per direct current ampere output is greatly less than that of other plate type rectifiers.

The principal performance characteristics of rectifier elements made in accordance with the invention are shown in the graphical representations of Figs. 2 and 3. Each curve is numbered to associate it with the rectifier element whose characteristics it depicts, as follows:

(1) A typical rectifier element formed by quenching the primary oxide film in a solution of nickel nitrate having a concentration of A molar and thereafter heating to 400 F. for 15 minutes.

(2) The process by which this element was made differed from that of element 1 only in that the concentration of the nickel nitrate quenchsolution for element 2 was one molar.

(3) No post-steaming treatment of any kind was used in making element 3, the counter-electrode being applied directly to the primary oxide film.

(4) This unit was the same as element 3, except that the primary film was subjected to the post-steaming heat treatment. There was no supplemental oxide deposition.

Invention is claimed as follows:

1. In a process of making a plate oxide rectifier, the step which comprises exposing a surface of commercial grade titanium to a stream of pure superheated steam moving at a velocity between 65 and 250 feet per minute at a temperature between 1400 F. to 1550 F. for a period of time sufficient to form a continuous film of oxide upon said surface.

2. In a process of making a plate oxide rectifier, the step which comprises exposing a surface of commercial grade titanium to a stream of pure superheated steam moving at a velocity between 65 and 250 feet per minute at a temperature between 1485 F. to l550 F. for a period of time sufficient to form a continuous film of oxide upon said surface.

3. In a process of making a plate oxide rectifier, the step which comprises exposing a surface of commercial grade titanium to a stream of pure superheated steam moving at a velocity between 65 and 250 feet per minute at a temperature between 1400 F. to 1550 F. for a period of from 30 minutes to 4 hours.

4. In a process of making a plate oxide rectifier, the steps which comprise exposing a surface of commercial grade titanium plate to a stream of superheated steam moving at a velocity between about 65 and about 250 feet per minute at a temperature between about 1200 F. and 1600 F. for a period of time sufficient to form a continuous primary oxide film upon said surface, and thereafter heating said plate in an inert atmosphere to a temperature between 300 F. and 600 F. and maintaining the temperature in this range for a period of several hours.

5. In a process of making a plate oxide rectifier, the steps which comprise exposing a surface of commercial grade titanium plate to a stream of superheated steam moving at a velocity between about 65 and about 250 feet per minute at a temperature between about 1200 F. and 1600 F. for a period of time sulficient to form a continuous primary oxide film upon said surface, heating said plate in an inert atmosphere to a temperature between 300 F. and 600 F. and maintaining the temperature in this range for a period of several hours, wetting the primary oxide film with a solution of a salt which is subject to thermal decomposition to an oxide which is an intrinsic semiconductor, and thereafter heating the plate to effect decomposition of the salt and deposition of the resulting oxide.

6. In a process of making a plate oxide rectifier, the steps which comprise exposing a surface of commercial grade titanium plate to a stream of superheated steam moving at a velocity between about 65 and about 250 feet per minute at a temperature between about 1200 F. and 1600 F. for a period of time sufficient to form a continuous primary oxide film upon said surface, heating said plate in an inert atmosphere to a temperature between 300 F. and 600 F. .and maintaining the temperature in this range for a period of several hours, wetting the primary oxide film with a solution of nickel nitrate, and thereafter heating the plate to effect decomposition of the nickel nitrate to nickel oxide.

7. In a process of making a plate oxide rectifier, the steps which comprise exposing a surface of commercial grade titanium plate to a stream of superheated steam moving .at a velocity between about 65 and about 250 feet per minute at a temperature between about 1200'F. and 1600 F. for a period of time sufficient to form a continuous primary oxide film upon said surface, heating said plate in an inert atmosphere to a temperature between 300 F. and 600 F.' and maintaining the temperature in this range for a period of several hours, quenching said plate in a solution of a salt which is subject to thermal decomposition to an oxide which is an intrinsic semiconductor, and thereafter heating the plate to effect decomposition of the salt and deposition of the resulting oxide.

8. In a process of making a plate oxide rectifier, the steps which comprise exposing a surface of commercial grade titanium plate to a stream of superheated steam moving at a velocity between about 65 and about 250 feet per minute at a temperature between about 1200 F. and 1600 F. for a period of time sufficient to form a continuous primary oxide film upon said surface, heating said plate in an inert atmosphere to a temperature between 300" F. and 600 F. .and maintaining the temperature in this range for a period of several hours, quenching said plate in a solution of nickel nitrate, and thereafter heating the plate to effect decomposition of the nickel nitrate to nickel oxide.

9. In a process of making a plate oxide rectifier, the steps which comprise exposing a surface of commercial grade titanium plate to a stream of superheated steam moving at a velocity between about 65 and about 250 feet per minute at a temperature between about 1200 F. and 1600 F. for a period of time sufiicient to form a continuous primary oxide film upon said surface, wetting the primary oxide film with a solution of a salt which is subject to thermal decomposition to an oxide which is an intrinsic semiconductor, and thereafter heating the plate to effect decomposition of the salt and deposition of the resulting oxide.

10. In a process of making a plate oxide rectifier, the steps which comprise exposing a surface of commercial grade titanium plate to a stream of superheated steam moving at a velocity between about 65 and about 250 feet per minute at a temperature between about 1200" F. and 1600 F. for a period of time sufficient to form a continuous primary oxide film upon said surface, wetting the primary oxide film with a solution of nickel nitrate, and thereafter heating the plate to effect decomposition of the nickel nitrate to nickel oxide.

11. In a process of making a plate oxide rectifier, the steps which comprise exposing a surface of commercial grade titanium plate to a stream of superheated steam moving at a velocity between about 65 and about 250 feet per minute at a temperature between about 1200 F. and 1600 F. for a period of time sufficient to form a continuous primary oxide film upon said surface, heating said plate and quenching it in a solution of a salt which is subject to thermal decomposition to an oxide which is an intrinsic semiconductor, and thereafter heating the plate to effect decomposition of the salt and deposition of the resulting oxide.

12. In a process of making a plate oxide rectifier, the steps which comprise exposing a surface of commercial grade titanium plate to a stream of superheated steam moving at a velocity between about 65 and about 250 feet per minute at a temperature between about 1200 F. and 1600 F. for a period of time sufiicient to form a continuous primary oxide film upon said surface, heating said plate and quenching it in a solution of nickel nitrate, and thereafter heating the plate to effect decomposition of the nickel nitrate to nickel oxide.

13. In a process of making a plate oxide rectifier, the steps which comprise exposing a surface of commercial grade titanium plate to a stream of superheated team moving at a velocity between about 65 and about 250 feet per minute at a temperature between about 1200 F. and 1600 F. for a period of time suflicient to form a continuous primary oxide film upon said surface, wetting the primary oxide film with a solution of a salt which is subject to thermal decomposition to an oxide which is an intrinsic semiconductor, and thereafter heating the plate in an inert atmosphere to a temperature between 300 F. and 600 F. and maintaining the temperature in this range for a period of several hours.

14. In a process of making a plate oxide rectifier, the steps which comprise exposing a surface of commercial grade titanium plate to a stream of superheated steam, moving at a velocity between about 65 and about 250 feet per minute at a temperature between about 1200 F. and 1600 F. for a period of time suflicient to form a continuous primary oxide film upon said surface, heating said plate and quenching it in a solution of a salt which is subject to thermal decomposition to an oxide which is an intrinsic semiconductor, and thereafter heating the plate in an inert atmosphere to a temperature between 300 F. and 600 F. and maintaining the temperature in this range for a period of several hours.

15. In a process of making a plate oxide rectifier, the step which comprises exposing a surface of commercial grade titanium to a stream of pure superheated steam moving at a velocity between'65 and 250 feet per minute at a temperature between 1400" F. to 1550 F. for a period of time sufficient to form a continuous film of oxide upon said surface, and thereafter heating said plate in an inert atmosphere to a temperature between 300 F. and 600 F. and maintaining the temperature in this range for a period of several hours.

16. In a process of making a plate oxide rectifier, the step which comprises exposing a surface of commercial grade titanium to a stream of pure superheated steam moving ata velocity between 65 and 250 feet per minute at a temperature between 1400 F. to 1550 F. for a period of time suflicient to form a continuous film of oxide upon said surface, heating said plate in an inert atmosphere to a temperature between 300 F. and 600 F. and maintaining the temperature in this range for a period of several hours, quenching said plate in a solution of a salt which is subject to thermal decomposition to an oxide which is an intrinsic semiconductor, and thereafter heating the plate to effect'decomposition of the salt and deposition of the resulting oxide.

17. In a process of making a plate oxide rectifier, the step which comprises exposing a surface of commercial grade titanium to a stream of pure superheated steam moving at a velocity between 65 and 250 feet per minute at a temperature between 1485 F. to 1550 F. for a period of time sufiicient to form a continuous film of oxide upon said surface, heating said plate in an inert atmosphere toa temperature between 300 F. and 600 F. and maintaining the temperature in this range for a period of several hours, quenching said plate in a solution of a salt which is subject to thermal decomposition to an oxide which is an intrinsic semiconductor, and thereafter heating the plate to eflect decomposition of the salt and deposition of the resulting oxide.

References Cited in the file of this patent UNITED STATES PATENTS Kruh May 12, 1908 

1. IN A PROCESS OF MAKING A PLATE OXIDE RECTIFIER, THE STEP WHICH COMPRISES EXPOSING A SURFACE OF COMMERCIAL GRADE TITANIUM TO A STREAM OF PURE SUPERHEATED STEAM MOVING AT A VELOCITY BETWEEN 65 AND 250 FEET PER MINUTE AT A TEMPERATURE BETWEEN 14000*F. TO 1550*F. FOR A PERIOD OF TIME SUFFICIENT TO FORM A CONTINUOUS FILM OF OXIDE UPON SAID SURFACE.
 5. IN A PROCESS OF MAKING A PLATE OXIDE RECITIFIER, THE STEPS WHICH COMPRISE EXPOSING A SURFACE OF COMMERCIAL GRADE TITANIUM PLATE TO A STREAM OF SUPERHEATED STEAM MOVING AT A VELOCITY BETWEEN ABOUT 65 AND ABOUT 250 FEET PER MINUTE AT A TEMPERATURE BETWEEN ABOUT 1200*F. AND 1600*F. FOR A PERIOD OF TIME SUFFICIENT TO FORM A CONTINUOUS PRIMARY OXIDE FILM UPON SAID SURFACE, HEATING SAID PLATE IN AN INERT ATMOSPHERE TO A TEMPERATURE BETWEEN 300*F. AND 600*F. AND MAINTAINING THE TEMPERATURE IN THIS RANGE FOR A PERIOD OF SEVERAL HOURS, WETTING THE PRIMARY OXIDE FILM WITH A SOLUTION OF A SALT WHICH IS SUBJECT TO THERMAL DECOMPOSITION TO AN OXIDE WHICH IS AN INSTRINSIC SEMICONDUCTOR, AND THEREAFTER HEATING THE PLATE TO EFFECT DECOMPOSITION OF THE SALT AND DECOMPOSITION OF THE RESULTING OXIDE. 